Television synchronization method and apparatus



April 14, 1936. R, LUBCKE 2,037,035

TELEVISION SYNCHRONIZATION METHOD AND APPARATUS Filed March 4, 1932 2 Sheets-Sheet l fad/a Trans/7127M I? g. .2. f

Harry R.Lubc7ce,

April 14, 1936, H R LUBCKE 2,037,035

TELEVISION SYNCHRONIZATION METHOD AND APPARATUS Filed March 4, 1932 2 Sheets-Sheet 2 Inventor; J'farvy R.Lubcke,

fia ,JW Attorney,

Patented Apr. l4, 1938 UNITED STATES PATENT OFFICE TELEVISION SYNCHBONIZATION METHOD AND APPARATUS 4 Claims.

My invention relates to television systems, and more specifically to means for synchronizing the scanning apparatus of television receivers and transmitters.

In television, it is necessary to have the scanning apparatus, at the receiver, and at the transmitter, operate exactly in step; so that the specific light intensity of a given spot in the field of view at the transmitter, will be reproduced in the same relative position-at the receiver. This requires coaction between the transmitter and receiver, in addition to the conveyance of information concerning the degree of illumination oi the elementary areas of the picture. This coaction is often obtained; by utilizing a second channel of communication from trarsmitter to receiver for the transmission of synchronizing pulses or frequencies; or by operating the two scanning means upon an alternating current power supply that is common to both. Each of these methods has disadvantages in comparison to one which is capable of transmitting all of the necessary in-= formation over a single channel of communication. To provide means for accomplishing this latter method, is one of the principal objects of the present invention.

Another object of this invention, possible of accomplishment when the nature of the scanning apparatus is non-mechanical, is to provide means for the retention of synchronization, although the receiving apparatus Figure 3 shows diagrammatically a complex I waveform that-is utilized at the receiver, for the removal of the return scanning trace;

Fig. 4 diagrammatically illustrates an alternative means for introducing a synchronizing pulse into the transmitted signal; and

Pig. 5 diagrammatically illustrates a modification of the last said alternative means,

7 Similar reference numerals refer to similar parts throughout the several views.

For the purpose of the present disclosure, it will be convenient to consider the use of my invention as applied to television transmission of a motion picture film.

Referring to Fig. 1, the illustrated cathode ray tube 88 provides means for producing a rapidly vibrating spot of light, for scanning a uniformly moving motion picture film 86. The detailed operation of this apparatus has been fully explained in my co-pending application No. 526,282, filed March 30, 1931. At 93 I have indicated a sensitive photoelectric cell, which is capable of receiving and responding to the instantaneous variations of light flux that occur in beams of light passing through the film, and of converting these variations into corresponding variations in an electric current. The modified current from the photoelectric cell is amplified, by means that include the thermionic vacuum tubes llll, and I02; and it then goes to the radio transmitter indicated at I03. The amplifying means need not be limited to the two vacuum tubes shown, and as many such tubes as are necessary may be employed, to bring the feeble photoelectric currents to a sulficiently high energy level for modulating the radio transmitter.

The function of the radio transmitter is to provide means for sending the amplified photoelectric current to a distant receiving station, and it may be of any suitable type known to the art. It should be noted also that, instead of the transmitter-receiver radio communication channel indicated, a single wire with earth return, or a pair of wires, may be used.

As illustrated in Fig. 1, the lower sprocket 88, which pulls the film through the projector mechanism, is so proportioned that it makes one complete revolution during the passage of four pic- 0 ture frames of the film past the scanning beam 81. A rotatable contactor I04, on the same shaft as the sprocket, and having four equally spaced insulating segments I05 upon its periphery, is provided; and this arrangement is adapted to cooperate with a brush I06, to open the circuit through the latter for brief instants, once for each picture frame that passes beam 81. Brush N16 is located on an angularly movable arm I 01, so that the time of the opening of the circuit can be made to occur at the same instant as the passage of the picture frame division lines past the constant horizontal level of scanning beam 81.

The circuit through brush I06, is normally closed, that is to say, the anode circuit through 2 elements I08--I09 of vacuum tube :02. rt Is opened, however, each time one of the segments I comes opposite brush I06. This causes the voltage developed at the top of resistor I00 to fall to zero; and it places a sharp and deep nega- I into space in the manner known to the art.

. ductor I00 will be positive,

the grid circuit of above, being similar The arrangement Just described may be used to operate on the anode circuit of any of the amplifying vacuum tubes; or upon their grid circuits, by changing the value of the grid bias; by methods that are well understood in the art. If the anode circuit of the next to the last tube is operated upon, the pulse appearing in conby virtue or the phase rotation characteristic of a vacuum tube. This will also be true of the pulse, if the bias of v the final vacuum tube is increased negatively to cut-off value.

A rotatable contacting device H0 is shown as being mechanically connected to the same shaft as sprocket 89 and contractor I04, as by the belt III. A four to one speed ratio is provided, so that contacting device H0 is adapted to perform the function of keeping the high frequency scanning source, consisting of vacuum tubes 4 I and 42, in step with the motion of the film. Incidentally the whole transmitting system may be kept in step with an cal arrangement for this purpose would consist in mounting the contacting device H0 on the shaft of a synchronous motor, with sprocket 09 and contactor I04 connected to it by a four to one speed reduction gear train.

The high frequency scanning source, ing of vacuum tubes 4| and 42 connected as shown, is adapted for producing asymmetricalpeaked, or saw tooth waves, by virtue of the small time constant of condenser-resistor com- I thus produce a short and steep rectangular pulse on one part of the cycle, while the large time constant of combination 4-! I5 causes the voltage at the top of inductor IlIi to be constant for the rest of the cycle. Inductor H6, in combination with resistor 'I I1, is made a high time-constant circuit. The above mentioned voltage wave, appearing at the top of inductor IIG, causes a saw-tooth wave of current to flow in the circuit. HIS-H1. By taking an output lead from the top of resistor III, a sawtooth wave of voltage is secured, since the current and voltage in and across a plain resistor are always in phase, and of proportional magnitude.

The saw-tooth voltage at the top of resistor H1 is applied to the deflection plate 30 of the scanning tube. It causes the electronic beam 25 to move relatively slowly from one side to the other, and then to rapidly return, with the saw-toot nature of the wave when plotted as a time function.

The high frequency scanning source described to the multivibrator of Abraham and Bloch, as described in 12 Ann. de Phys. 237, 1919, has a characteristic sensitivity to synchronization by an outside pulse. To obtain such a pulse, the rotatable contacting device H0 is provided with a single conducting contact point 8- on its periphery. This connects the brush H9 to ground, once for eachrevolution of alternating current power. supply in this manner; and a preferred mechaniconsist- 'in accordance the contacting device. The time required for one revolution of the contacting device is the same as that required for the passage of one frame of the film past the scanning beam 81. Since brush I I9 is connected to the grid of tube 4|, it grounds it once per picture, for a time that is small in comparison to one cycle of the scanning source. Although the scanning source may execute, say, eighty complete cycles, to everyv synchronizing pulse, its inherent stability is such that there is no noticeable tendency for it to drift in frequency. A battery I20 can be inserted in the circuit as shown, which will instantaneously bring the normal potential of the grid to any value, positive or negative, according to its size and polarity, to insure synchronization in all cases.

The lead from brush I I0 can also be connected to a second grid; or to the top of the anode resistor I2l; or to inductor H0 in certain cases, to secure synchronization. The phase angle between an alternating current power line, and the scanning source, is subject to being changed by this expedient.

An alternative arrangement, that may be substituted for contactor I04 and brush I06, consists in having two conducting contact points 8, insulated from each other, but at the same angular position upon contacting device I II. Two brushes I I9 would be used, one being in cooperation with contacting device H0, as'shown, and the other brush and contact taking the place of contactor I04 and brush I08. The brushes in such a case, must be so arranged that the circuit corresponding to that through the contactor I04 and brush I06 shall normally be closed; and be opened only at the proper time, as previously described.

The above alternative arrangement has the advantage of providing means foixqbtaining a synchronizing pulse in the image signal, of a fixed phase, and which is adapted to synchronize the high and low frequency receiver scanning sources simultaneously. The pulse derived from contacting device H0, lasts only a fraction of the time of a cycle of the high frequency scanning, and is repeated once each cycle of the low frequency scanning.

- 'A still further arrangement for introducing the synchronizing pulse is possible, when the well known Nipkow disk is used at the transmitter. Such an arrangement is illustrated in Fig. 4. The disk II, with a circularly arranged ring of holes I2 and a light source I3 arranged at the left of it, performs the function and takes the position of the cathode ray tube 88 in Fig. 1. It is usual practice to make the mask or shield I4 that restricts the area illuminated by the light source, of such size that only one hole of the disk appears in it, and passes light through the film 06 and to the photoelectric cell 93 at any instant of time. a

By ma mg the mask slightly larger circumferentially, however, it is possible to produce a synchronizing pulse in the image signal. If the opening in the mask is made sufficiently larger to take in another disk hole, as shown in Fig. 4, it will be seen that light will pass from the source through the film and to the photoelectric cell 83 from two holes, for a brief instant at the completion of one scan and the start of the next. This will cause a current response from the photoelectric cell twice as great as the maximum response possible during the rest of the scan, assuming that there is a light background, such as is usually used for television presentations.

This gives a pronounced pulse of constant p and of a period equal to that of the pulse conveying the elementaliy sized change in intensity in the image. By making the mask still larger a pulse ofany desired period can be obtained; at the expense, however, of part of the time available for useful scanning.

The low frequency synchronizing pulse can likewise be inserted by partially sacrificing one of the scanning lines in the image. one hole in the disk may be made two or more times as large in area as any'of the rest as indicated in Fig. 4. As this hole passes across the aperture in the mask the light that reaches the photoelectric cell is correspondingly greater, and, in relation to the rest of the scans across the image, there is a pulse of a period equal to the time of one scan. The pulse of this periodJs filtered out at the receiver in a manner that will be explained later. The scan used for this operation should be the first in the image, since this is usually the sky or otherwise the lightest part. This can be accomplished by proper framing at the transmitter. l

It is also evident that the black frame line of the motion picture film gives a pulse. of simi lar duration but of zero amplitude, causing it to be a negative pulse in the image signal. This pulse can also be filtered out at the receiver.

Also, in case a spiral of holes is used in the transmitting disk, for scanning a subject, as il lustrated in Fig. 5 at I6, the low frequency pulse can be inserted by placing a notch I1 at the upper or lower extremity of the aperture of the mask, enlarging the aperture circumferentially in this manner to allow two scanning holes to traverse the field of view instantaneously; as,

considered previously, for high frequency synchronization. If the enlarged mask is used for high frequency synchronization, the notch will still provide independent low frequency synchronization, since it will produce a pulse of greater period than the frequently recurring high frequency pulse, and thus can be filtered out at the receiver.

Passing now to a consideration of the receiving apparatus, as shown diagrammatically in Fig.2, a radio receiver is conventionally shown at I22. This apparatus is capable of: intercepting the emissions of transmitter I 03;'demodulating (or detecting) them; and amplifying them if necessary, to the end that substantially the same electric pulses will travel in wires I23 and I24, that travel in wire I00. at the transmitter. It is convenient to have wire I23 carry the full output of the receiver, and to have wire I24 carry only a fraction of that output. This resuit can be secured by having wire I24 connect with a tap on a potentiometer arrangement of resistors, as indicated in the figure.

Wire I23 is connected to the grid, or control electrode I25 of the receiving cathode-ray osciilograph tube I26. The intensity of the electron beam I21 is thus controlled, resulting in producing a fluorescent spot at I28, on fluorescent screen I29, the intensity of which varies according-to the image pulses in wire I23, and thus according to the light intensity of the corresponding elementary area of the film 86, where scanned by beam 61 of the transmitting apparatus.

Wire I24 connects to the grid circuit of vacuum tube I30, which, in combination with the other elements connected thereto, comprises what I choose to call a selecting amplifien. As shown,

it embraces means for the selection of the low frequency framing pulse, originated at switch I06. This comprises resistor I3I and condenser I32. This pulse is conveyed to the low frequency scanning source, comprising vacuum tubes 15A, 15, and 18 by wire I33. It also includes means for selecting the high scanning frequency pulse from the image signal, by means of the tuned transformer comprising elements I34, I35, I36, and I31, which pulse is conveyed to the high frequency scanning source by wire I33.

The action of the elements in the selecting amplifier may be described as follows:-

The portion of the signal fed to oscillograph tube I26 is amplified in a distorted manner, because of the coaction of the elements in the anode circuit of the selecting amplifier. The tuned transformer, consisting of condenser I34, primary transformer coil I35, secondary transformer coil I 36, and condenser I31, is arranged to provide a high-impedance for frequencies at or near the high scanning frequency, by properly choosing the values for the various components, in ways well known to the art. At other frequencies the voltage built up across coil I35, and subsequently transformed in coil I36, is negligible. Thus the pulse at the scanning frequency and phase that made the image, is recovered for synchronizing the receiver source, in a manner similar to the way the transmitter scanning source was synchronized to an alternating current power supply, as described above.

In the arrangement shown, the pulse created at switch I06 is of longer duration than any high frequency scanning pulse. Resistor I3I represents an impedance to this frequency, although not to the high scanning pulse, since the latter is lay-passed by a condenser 132 of proper size. Thus the low frequency synchronnzing pulse appears at top of resistor I3I, and is applied to the low frequency scanning source by wire I33. This source may be exactly similar to that shown in Fig. l of my said co-pending application No. 526,282; and is synchronized in the same manner as the high frequency receiver source previously referred to.

By means of varying the capacitance of condenser I31, the phase of the high frequency pulse can. be changed, to place the received image in the center of the-field of view of the receiver. This can also be accomplished by changing the value of condenser I34.

If the two-contact arrangement of contacting device H is used, to give one pulse for both high and low frequency synchronization, only a tuned transformer arrangement is required; both of the wires I33 and I38 being connected to the top of coil I36.

The circuit consisting of elements I39, I40, I4I, I42, I43, I44 and I45 is provided to remove the unused return scan of the high frequency scanning source, from the fluorescent screen I29, by suitably affecting the vertical deflecting plates 28 and 29.

As previously mentioned, the voltage wave appearing at top of inductance I46 of the receiver is rectangular, and of unequal periods corresponding exactly to the time of the across and return scans which it produces. Condenser I45 and primary coil I44 provide a high impedance shunt circuit to the scanning sourcei for securing a portion of the total amplitude appearing across inductance I46, so as to appear across coil I44 without shorting the normal action of the scanning source. Secondary coil I42 gives a pulses; said impedances stepped-up magnitude to the wave, with resistor I43 placed across it to attentuate any oscillatory transients that may occur in the waveform.

The normal, saw-tooth wave, low frequency output, appears across resistor I39. It is thus seen that the output of the combination of elements 2 and I43 is superposed upon it.

Fig. 3 shows the resulting waveform. Battery I" is provided to bias the plates 28 and 29, in such a manner that the used scans, represented in time by interval H of Fig. 3, are located on the fluorescent screen; while the return scans represented in time by interval b-c of Fig. 3, are located off the screen. The amplitude (c-d of Fig. 3) of the output of coil 2 is made great enough to completely remove the return scan traces from the screen. way, is a Lissajous figure consisting of, over a period of one cycle of the low frequency, a rectangular group of used scans on the fluorescent screen, which display the image, plus a similar group of return scan traces, executed oi! the screen and hence never seen, with vertical connecting lines between a used scan and its successive return scan trace, which are so faint because of the speed with which they are executed, that they do not visually appear.

Having thus full described my invention, in a manner that will be understood by those having knowledge of the art, I claim:

1. In television apparatus, means adapted for selectively utilizing a plurality of pulses for synchronizing scanning, comprising; a thermionic amplifier having impedances in its anode circuit;

I and means for varying the phase relation between said synchronization pulses and the scanning being relatively great at the frequency of said synchronizing pulses.

2. In television apparatus, means for producing a quasi-rectangular synchronizing pulse of substantially constantamplitude in. a television signal, comprising; a contactor connected in the anode circuit of an image signal amplifier, between the source of constant energizing voltage and the adjacent terminal of the external anode impedance, whereby the anode supply voltage of said amplifier may be periodically varied; and means for operating the contactor to accomplish said periodical variation.

3. In the art of television, an operating method which includes; producing, at a transmitting station, two periodic quasi-rectangular pulses of materially different frequency; transmitting said pulses to a receiving station via an image-signal channel; separating said pulses from said channel at the receiving station by means of two impedances respectively, the irfipedances being connected in series in the anode circuit of a single thermionic device; applying said selected pulses respectively to two self-oscillating sources of rectilinear scanning waveshapes, to initiate each successive waveshape produced thereby; and then utilizing said waveshapes co-operatively in a cathode ray tube, for reproducing an image transmitted over said channel; each of said impedances being adapted for giving a. maximum response at the frequency of one of said pulses respectively, and a minimum response at the frequency of the other said pulse.

4. In the art of television, an operating method for the transmission of motion picture filmwhich includes; producing synchronizing signals of zero intensity at a transmitting station between the scannings of successive picture areas of the film, by scanning the division lines between said areas; producing image signals at said station, corresponding to the opacity along successive scanning traverses across the picture areas; transmitting all of said signals to a receiving station over a single channel of communication; separating the synchronizing signals from the image signals at the receiving station, through selection of their materially different frequencies, by means of two impedances respectively, the impedances being connected in series in the anode circuit of a single thermionic device; applying said selected signals respectively to two self-oscillating sources of rectilinear scanning waveshapes, to initiate each successive waveshape produced thereby; and then utilizing said waveshapes co-operatively in a cathode ray tube, for reproducing an image transmitted over said channel; each of said impedances being adapted for giving a maximum response at .the frequency of one of said signals respectively, and a minimum response at the frequency of the other said signal; the frequency of said synchronizing signals corresponding to the time required for scanning a complete picture area and a single division line between picture areas; and the frequency of said image signals corresponding to the time required for each scanning traverse across the picture areas.

HARRY R. LUBCKE. 

